An outside investigation into Dan Rather’s bogus National Guard documents story will take only “a few weeks” – not months – and the head of CBS’s parent company said get ready for heads to roll.

Sumner Redstone, chairman and chief executive of Viacom, spoke for the first time yesterday about the scandal.

The investigation is “appropriate – and the consequences will be appropriate,” Redstone told the business magazine Forbes at a conference in Hong Kong.

As network officials were privately informing reporters they would like to get to the bottom of the scandal before the November election, one of the two investigators appointed this week – former Pennsylvania Gov. Dick Thornburgh – was himself becoming an issue.

Rather – whose botched Sept. 8 report on President Bush’s Vietnam-era service in the Texas National Guard sparked the controversy – is reportedly upset at the appointment of Thornburgh, a Republican who served as attorney general under Presidents Ronald Reagan and George H.W. Bush.

“I’m sure he will do a solid and professional job of investigating this problem,” added Kalb, who worked with Thornburgh at Harvard’s Kennedy School.

While sources said the network’s investigation will be wrapped up in a few weeks, a CBS spokeswoman insisted there was no deadline.

“Everyone at CBS would like it done as quickly as possible but the most important thing is that it be done as thoroughly and as completely as possible,” CBS News spokeswoman Sandra Genelius said.

Rather on Monday apologized on his nightly news program for the “60 Minutes” report, which was based on a late officer’s “memos” that CBS has admitted could not be authenticated. Since then, sources at CBS have said that the segment’s producer, “60 Minutes” veteran Mary Mapes, had misled her bosses about the reliability of the documents’ source.